The overall objective of the present proposal is to delineate the role of endogenous opioids in regulating parental behavior in mammals. Using a rat model, a series of studies are proposed to test the hypothesis that endogenous opioids are stimulatory to parental behavior. A combination of behavioral, physiological, and anatomical approaches will be employed to test this hypothesis. In the initial series of studies adult and juvenile rats will be treated with opiate antagonists and agonists to evaluate whether blocking opiate activity will delay and increasing opiate activity enhance the expression of parental care. To provide a biological basis for assessing the actions of endogenous opioids in the induction of parental care proopiomelanocortin (POMC) mRNA (the precursor for B-endorphin) and opiate receptor (u) mRNA expression in brains of adult female and juvenile female and male rats will be measured by in situ hybridization histochemistry (ISHH). In addition, beta-endorphin release by push-push perfusion (PPP) will be quantified during late pregnancy and at parturition. A second study will evaluate the role of opioids in the establishment of "maternal memory" by blocking central opioid activity during the immediate postpartum period when this memory is established. The final set of studies will attempt to clarify the role of opioids during lactation in maternal behavior. Behaviors will be monitored in lactating rats after treatment with opiate antagonists to test the proposal that opioids released during nursing help quiet the mother and keep her with the young. Measurements of neural mRNA expression of POMC and the u-opiate receptor by ISHH, determination of opioid release by PPP, and quantification of neural opioid receptor distributions and densities by autoradiography throughout lactation will provide the basis for assessing the physiological role of endogenous opioids in ongoing maternal behavior. Together, these studies will contribute to understanding of the neurobiological underpinnings of parental behavior, and thus provide a framework for evaluating the effects of exogenous opiates and stressful environmental stimuli which alter endogenous opioid activity on parent-young interactions.